INTRODUCTION

Aspects of geology, geomorphology, and evolution and resource potential of the Arctic shelf attract close attention of researchers. However, the degree of knowledge of bottom sediment sequence of the Russian Arctic seas remains extremely uneven. The coastal areas of the East Siberian Sea are among the most poorly studied regions. Until recently, the water area of the R-58-60 sheets of the state geological 1 :  1000 000 scale map was literally a “blank spot.” The available data on seismo-acoustic profiling for the studied area are extremely limited. Experimental geological and geophysical works using this method were carried out in 1987–1988 in Kolyma Bay. In 2010, Sevmorgeo carried out the regional profile 5AR (west of Wrangel Island). The only relatively deep borehole (650 m) in the study area was drilled on Ayon Island [1].

A number of unpublished reports of the 1970s–1980s and articles contain limited available data on the upper sediment sequence [7, 9–11]. The lack of seismo-acoustic profiling data and radiocarbon datings of the Quaternary sediments caused ambiguity in the interpretation of the regional Quaternary evolution.

MATERIALS AND METHODS

The coastal areas of the East Siberian Sea from the New Siberian Islands to Wrangel Island were covered by the State Geological Mapping (GK-1000/3), sheets R56-60 and S-55,56 during the expeditions of the Karpinsky Russian Geological Research Institute in 2018 and 2020. A total of 3400 km of continuous seismo-acoustic profiling (CSAP) with the simultaneous use of several methods (sparker, seismic airgun, seismic profiler) and 3200 km of profiling by side-scan sonar (SSS) and multibeam echo sounding have been carried out. Sampling of sea-floor sediments was fulfilled at 191 stations with a box corer (Fig. 1); 29 sediment cores (0.25 to 2.5 m long) were recovered with a gravity corer (Fig. 2).

Fig. 1.
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The scheme of the field research activities of the Karpinsky Russian Geological Research Institute in 2018 and 2020: 1, site locations of sediment cores; 2, profiles of multi-frequency CSAP.

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Fig. 2.
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Photographs and lithologic composition of the sediment cores: 1, fine-grained sand; 2, mictite (silty clay with sand grains, single gravel, and pebbles); 3, sandy-clayey silt with single gravel; 4, clayey silt; 5, silty clay; 6, fragments of mollusk shells; 7, admixture of fine-grained sand; 8, biogenic textures (burrows of mud-eaters); 9, pebbles and gravel; 10, plant debris; 11, large folds with “marble-like” seafloor erosion by ice ridges 12, laminated; 13, massive mottled, heterogeneous (mixed); 14, stiff.

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In 2022, 11 sediment cores were analyzed at the Shirshov Institute of Oceanology, RAS, with the use of the automated system for the integrated core study Geotek MSCL-XYZ (scanning, photography, geochemical studies). The results of XRF-analysis were smoothed using a moving average calculation (window = 7). Detailed grain-size analysis of 29 sediment cores with 1-cm resolution was conducted in VSEGEI using the Microtrac MrB laser particle size analyzer. Six cores were analyzed for palynology and diatoms. For the first time, 13 radiocarbon dates of scattered organic matter from seafloor sediments were obtained in the Laboratory of Radiocarbon Dating and Electron Microscopy of the Institute of Geography, RAS, and in the Center for Applied Isotope Research of the University of Georgia (United States) (Table 1). The concentration of organic carbon (Corg) was determined on the AN-7529M express carbon analyzer by the automatic coulometric titration on the basis of the pH value in the Laboratory of Atlantic Geology, Shirshov Institute of Oceanology, RAS.

Table 1. Radiocarbon dating of obtained sediment cores (material for radiocarbon dating–scattered organic matter)
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Marine navigation 1 : 500 000 and 1 : 200 000 scale maps were used to create a digital bathymetric model necessary for the geomorphological analysis and interpretation of geological and geophysical data when compiling the map of the Quaternary formations and reconstructing paleogeographical conditions. The points of depth measurements and isobaths were digitized in ArcGIS program; the interpolation was carried out by the “Topo to Raster” method with the use of the ArcGIS spatial analyst module.

RESULTS AND DISCUSSION

According to the CSAP and core sampling, five seismo-stratigraphic units (SSUs) of the Quaternary sediments were identified in the generalized section, which, in turn, can be subdivided into local subunits. The GIS schemes of the top and thickness of the selected SSUs along the profiles were plotted. The preliminary seismo-stratigraphic scheme of the Quaternary sediments of the East Siberian Sea (from the New Siberia Islands to Wrangel Island) was compiled.

It should be noted that in the absence of stratigraphic drilling in the studied area, the age interpretation of the lower seismic formations is based on the evidence from the adjacent land. This interpretation is far from unambiguous and, obviously, requires further research.

When creating the unified seismo-stratigraphic scheme of the southern East Siberian Sea, we relied on the available data on marine sedimentary cores and on the results of the studies performed on islands, mainland [2, 3, 5], and adjacent areas of the Chukchi Sea seafloor [6]. We also considered the published data on the outer shelf of the East Siberian and the Laptev seas [13], materials of the state geological survey of the land and islands, and the unpublished (archive) reports of 1971–1988 on prospecting geological works.

At the current level of knowledge, the seismostratigraphic scheme is as follows.

In the western part of the area, SSU5 is characterized by a Gelasian–Lower Pleistocene age; and in the eastern part, by the Gelasian (?)–Lower Pleistocene age, supposedly with predominance of marine sediments. Lacustrine and alluvial beds are also present. SSU4 is represented by the Lower (?)–Middle Pleistocene lacustrine, alluvial, and marine sediments. SSU3 is represented by the Upper Pleistocene marine sediments (Kazantsevo transgression), lacustrine sediments or a hiatus in sedimentation (Zyryanka regression). SSU2 is represented by the Upper Pleistocene, marine sediments (Karginsky warming); it is correlated with the Molotkov horizon of the yedoma suite on land. SSU1 corresponds to the Upper Pleistocene (Sartan cooling)–Holocene (post-Sartan warming–sea ingression), marine sediments of different facies. SSU1 deposits characterize the successive change of lacustrine (generally thermokarst), coastal-marine, lagoonal, and marine sediments. The distinguished SSUs characterize the cyclic sedimentation since the end of the Middle Pleistocene (Figs. 3, 4).

Fig. 3.
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Seismo-acoustic profile 0066-1_10: (a) the time section obtained using high-frequency profiler; (b) the time section obtained using sparker seismic source. Chronostratigraphic interpretation of SSUs is in the text.

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Fig. 4.
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Seismo-acoustic profile ESS-07_ESS-8: (a) the time section obtained using a high-frequency profiler; (b) the time section obtained using airgun seismic source. Chronostratigraphic interpretation of the SSUs is in the text.

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Sediment cores penetrated sediments of SSU4–SSU1. The upper parts of SSU4–SSU2 represent sediments that were subaerially exposed during coolings and regressions in the Middle Pleistocene, and the Late Pleistocene Zyryanka and Sartan cold epochs. They are composed of extremely dense sediments, as a result, the obtained cores are short in length.

The recovered sediments of SSU4 (Lower (?)–Middle Pleistocene lacustrine, alluvial and marine) (cores 20BCM-46, 20BCM-47)Footnote 1 indicate an unstable sedimentation in environment of predominantly freshwater reservoir with low bioproductivity in close proximity to sediment source areas.

The cores that penetrated the SSU3 upper horizons (Upper Pleistocene, Kazantsevo transgression–Zyryanka regression) (20BCM-35 (20–0 cm), 20BCM-79 (130–78 cm), 20BCM-69, 20BCM-70, and 20BCM-71 (20–0 cm)) also recovered sediments characteristic of freshwater environment and low bioproductivity, but have a finer-grained composition. The cores that sampled sediments of SSU2 [Upper Pleistocene (Karginsky transgression–Sartan regression) (20BCM-37 (23–0 cm), 20BCM-1 (20–0 cm) and 20BCM-4 (37–0 cm) and 18BCM-69 (100–80 cm)) indicate active hydrodynamic sedimentation environment, low salinity of the paleobasin, and stable hydrochemical characteristics.

The SSU1 sediments were sampled with the largest number of sediment cores. The selection of sites along the CSAP profiles allowed us to obtain material for the studies of different time slices and, thus, to trace the development of the post-Sartan transgression. Sediment cores 18BCM-3 (De Long Strait), 18BCM-96, and 18BCM-97 (Kolyma River paleovalley) are the most informative to characterize the lower part of SSU1.

Sediments of core 18BCM-3 (interval 158–38 cm) differ sharply in their characteristics from other cores from this area representing the Holocene deposits [17]. The unit is characterized by distinct subhorizontal lamination of silty clayey sediments, not disturbed by traces of benthic activity or ice scours and is enriched in Corg (2%). Parameters of the grain-size composition of sediments in this interval are relatively constant; the share of pelitic particles exceeds 50% indicating hydrodynamically calm bottom environment. The sediment unit also differs from the Holocene sediments in geochemical parameters. Pollen spectra of core 18BCM-3) contain pollen of coniferous trees, which could have been transported with freshwater streams from the southern regions. According to diatom analysis, freshwater diatoms predominate in the sediments of the 156–112 cm interval and benthic brackish-water diatoms are present, which can be interpreted as the lake or lagoonal conditions (with a noticeable freshwater inflow) [17].

Similar conclusions can be drawn from the results of paleosalinity calculations using the distribution of bromine concentrations in the sediments. A date of 18 584 cal. yrs. BP (IGANAMS 7551) was obtained for core base (Table 1), which is likely too old due to the input of ancient organic matter, but in any case, characterizes the time range between the beginning of the Late Pleistocene warming and the time when the marine transgression reached the core site. According to the complex of the features described, the sediments of this part of the core correspond to sedimentation in a thermokarst lake.

The transition to the coastal-marine (lagoonal?) conditions in core 18BCM-3 is clearly manifested by all proxies (paleosalinity, diatom association, geochemical parameters) in the 0- to 30-cm interval [17]. The mechanism of replacement of lake-thermokarst sediments by lagoonal sediments due to thermoabrasion of shores, which to some extent is analogous to the processes that occurred during the transgression, has been traced on the modern Arctic coasts.

The shallow-marine sedimentation (with a variety of facies) characterizes cores18BCM-96 (33–0 cm), 18BCM-97 (65–0 cm), 18BCM-12, 18BCM-105, 20BCM-8 (175–35 cm), 20BCM-9 (175–40 cm), 20BCM-11, 20BCM-12, 20BCM-13 (74–0 cm), 20BCM-26, 18BCM-3 (38–0 cm), 20BCM-66, 18BCM-37, 18BCM-17, and 18-BCM-18. The Late Holocene typical marine sediments were sampled in the upper horizons of cores 20BCM-8 (35–0 cm) and 20BCM-9 (40–0 cm), as well as in the surface samples collected by a box-corer.

Our main task is to develop a consistent age model taking into consideration all the data obtained and the general concept of the paleogeographical development of the eastern Arctic seas and New Siberian Islands in the Late Pleistocene and the Holocene. From this point of view, detailed study of core 20BCM-8, which was selected as one of the reference cores, is extremely important. The core was taken in the deepest part of the Indigirka River paleovalley. In the basal part (177–178 cm) the radiocarbon dating of 18 690  cal. yrs BP (IGANAMS 8975) was obtained (Table 1). However, the entire core section was accumulated in the Holocene, as is evident from both the character of the seismo-acoustic record (the SSU1 thickness according to the CSAP data at the 20BCM-8 site is about 5 m) and the results of the geochemical and grain-size studies (Fig. 5), indicating nepheloid sedimentation under the coastal-marine (brackishwater) conditions (the core base) with gradual transition to typical marine sedimentation.

Fig. 5.
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Results of layer-by-layer analytical studies of sediment core 20BCM-8.

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Study of microfossils revealed that the core sediments contain rare benthic foraminifera and ostracods, especially in the upper 60 cm, where carbonate tests and valves are partially dissolved. Arctic shallow-water river-proximal species predominate. These are benthic foraminifers Haynesina orbiculare, Elphidium bartletti, E. incertum, Elphidiella groenlandica, Buccella frigida, and Polymorphina spp. and the euryhaline ostracods Paracyprideis pseudopunctillata and Heterocyprideis sorbyana. Shells of the Arctic bivalve species Portlandia arctica are abundant throughout the core.

Analysis of the morphology and stratigraphic and bathymetric position of the ridges located in the coastal shallows on the periphery of New Siberia Island (Fig. 6), as well as the composition and parameters of the grain-size composition of sediments, allowed us to distinguish two different types of ridges in terms of age and genesis, that is, the accumulative coastal-marine ridges (submarine bars), and the ridges formed under the influence of currents (under submarine conditions) or erosional ridges (under subaerial conditions) [11]. In the course of our research, glacial landforms were not established.

Fig. 6.
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Submarine landforms around New Siberia Island: (A) three-dimensional panorama of the digital model of the seafloor topography with the position of some CSAP profiles (red lines are fragments of profiles shown in Fig. 6B); (B) fragments of high-frequency CSAP profiles (profiler) crossing submarine bars and ridges of different origin.

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Preliminary paleogeographical reconstructions of the regional evolution during the last 24 000 years were created on the basis of the constructed models of seafloor topography and the existing conceptions about sea-level change [13] (Fig. 7). The reconstruction illustrates the asynchrony in the post-Sartan transgressive flooding of different regions of the East Siberian shelf. The studied area belongs to the zones of low and moderate geodynamic activity [8]. During the last (Sartan) cooling under predominantly continental conditions processes of erosion and thermoerosion prevailed. No landforms or sediments, formed as a result of glacial activity, have been found in the coastal zone, which confirms the absence of continental glaciation on Wrangel Island and the New Siberian Islands during the Last Glacial Maximum. This confirms, in particular, the conclusions of [5]. This work convincingly proves the existence of a glacier on New  Siberia Island only at the end of the Middle Pleistocene.

Fig. 7.
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Time-scale paleogeographical reconstructions of the Late Pleistocene–Holocene evolution of the southern East Siberian and the Chukchi seas and corresponding sea-level position.

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In the Late Pleistocene–Early Holocene, the coastal areas of the East Siberian Sea were subaerially exposed. During warming, these land areas were subject to thermokarst processes, permafrost degradation, active erosion and sediment accumulation in shallow freshwater thermokarst lakes. Flooding of the shelf proceeded very unevenly from east to west, starting from the Chukchi Sea, where a bay was formed around 11 cal.ka, while the rest of the study area remained exposed. Around 10 cal.ka, the first lagoons associated with the open sea were formed west of Wrangel Island. By 8.5 cal.ka, the modern coastal areas of the Chukchi Sea and the eastern part of the East Siberian Sea had already been flooded by the sea (Fig. 7), while in the western part of the East Siberian Sea, the land still extended to the modern New Siberia Island.

These paleoreconstructions are confirmed by the published results of the study and dating of large mammal bones found on Zhokhov, Bennett, and Wrangel islands [2, 4]. Around 8.5 cal.ka, ingression sea bays were formed in the area of the Indigirka and Kolyma paleodeltas, as well as in the estuary of a river that  flowed into the sea to the north of the modern Chaun Bay.

Marine transgression developed rather unevenly, as evidenced by characteristic landforms (coastal bars, submarine terraces, foredeltas, etc.). The accumulation rate of the Holocene and modern marine sediments in the studied area is usually very low (except for submarine paleovalleys), which is proved by the limited thickness of marine sediments sampled by gravity cores. The stratification of the Holocene sediments is significantly disturbed due to the widespread impact of drifting ice on the seafloor.

CONCLUSIONS

Five seismic units were distinguished in the Quaternary sediment sequence of the coastal East Siberian Sea on the basis of seismo-acoustic profiling and geological sampling carried out in 2018–2020. The established units generally reflect the main stages of the paleogeographical development of the region.

Paleogeographical reconstructions showed that at the end of the Late Pleistocene–Early Holocene, the coastal areas of the East Siberian Sea represented land where complex thermal-denudation processes developed during warming, while sedimentation occurred under continental conditions. Transgression of sea waters occurred from east to west, and around 10 cal.ka, the first lagoons, connected with the open sea, were formed west of Wrangel Island. Around 8.5 cal.ka, in the western part of the East Siberian Sea, the land extended to the modern New Siberia Island, and there was a deep ingression bay at the place of the Indigirka paleovalley, while the rest of the shelf was already flooded by the sea.

For more reliable stratigraphical and geochronological attribution of the lower seismic units, it is necessary to drill and survey boreholes on the seafloor. In 2022, borehole DL-1 was drilled by the Karpinsky Russian Geological Research Institute jointly with Rosneft, Rosgeo, and AMIGE to the north of New Siberia Island. Detailed study of the obtained sediment core will allow us to continue the initiated research and to clarify the conceptions about the geological structure and paleogeography of the region.